The present invention relates generally to means and method for a linear, multipole ion trap whereby ions from an ion source are transmitted through a differential pump system and into a multipole trap device for trapping and analysis. More specifically, an apparatus for a linear quadrupole trap is described which uses one multipole device comprising two trapping regions and one analyzing section to provide an improved mass analyzer.
The present invention relates generally to a multipole ion trap for use in mass spectrometry. The methods for transferring, trapping and analyzing ions described herein are enhancements of the techniques referred to in the literature relating to mass spectrometry.
Mass spectrometry is an important tool in the analysis of a wide range of chemical compounds. Specifically, mass spectrometers can be used to determine the molecular weight of sample compounds. The analysis of samples by mass spectrometry consists of three main stepsxe2x80x94formation of gas phase ions from sample material, mass analysis of the ions to separate the ions from one another according to ion mass, and detection of the ions. A variety of means exist in the field of mass spectrometry to perform each of these three functions. The particular combination of means used in a given spectrometer determine the characteristics of that spectrometer.
To mass analyze ions, for example, one might use a magnetic (B) or electrostatic (E) analyzer. Ions passing through a magnetic or electrostatic field will follow a curved path. In a magnetic field the curvature of the path will be indicative of the momentum-to-charge ratio of the ion. In an electrostatic field, the curvature of the path will be indicative of the energy-to-charge ratio of the ion. If magnetic and electrostatic analyzers are used consecutively, then both the momentum-to-charge and energy-to-charge ratios of the ions will be known and the mass of the ion will thereby be determined. Other mass analyzers are the quadrupole (Q), the ion cyclotron resonance (ICR), the time-of-flight (TOF), and the quadrupole ion trap analyzers.
Before mass analysis can begin, however, gas phase ions must be formed from sample material. If the sample material is sufficiently volatile, ions may be formed by electron impact (EI) or chemical ionization (CI) of the gas phase sample molecules. For solid samples (e.g. semiconductors, or crystallized materials), ions can be formed by desorption and ionization of sample molecules by bombardment with high energy particles. Secondary ion mass spectrometry (SIMS), for example, uses keV ions to desorb and ionize sample material. In the SIMS process a large amount of energy is deposited in the analyte molecules. As a result, fragile molecules will be fragmented. This fragmentation is undesirable in that information regarding the original composition of the samplexe2x80x94e.g., the molecular weight of sample moleculesxe2x80x94will be lost.
For more labile, fragile molecules, other ionization methods now exist. The plasma desorption (PD) technique was introduced by Macfarlane et al. in 1974 (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F., Biochem. Biophys. Res Commoun. 60 (1974) 616). Macfarlane et al. discovered that the impact of high energy (MeV) ions on a surface, like SIMS would cause desorption and ionization of small analyte molecules, however, unlike SIMS, the PD process results also in the desorption of larger, more labile speciesxe2x80x94e.g., insulin and other protein molecules.
Lasers have been used in a similar manner to induce desorption of biological or other labile molecules. See, for example, VanBreeman, R. B.: Snow, M.: Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Tabet, J. C.; Cotter, R. J., Anal. Chem. 56 (1984) 1662; or Olthoff, J. K.; Lys, I.: Demirev, P.: Cotter, R. J., Anal. Instrument. 16 (1987) 93. Cotter et al. modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. The plasma or laser desorption and ionization of labile molecules relies on the deposition of little or no energy in the analyte molecules of interest. The use of lasers to desorb and ionize labile molecules intact was enhanced by the introduction of matrix assisted laser desorption ionization (MALDI) (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T., Rapid Commun. Mass Spectrom. 2 (1988) 151 and Karas, M.; Hillenkamp, F., Anal. Chem. 60 (1988) 2299). In the MALDI process, an analyte is dissolved in a solid, organic matrix. Laser light of a wavelength that is absorbed by the solid matrix but not by the analyte is used to excite the sample. Thus, the matrix is excited directly by the laser, and the excited matrix sublimes into the gas phase carrying with it the analyte molecules. The analyte molecules are then ionized by proton, electron, or cation transfer from the matrix molecules to the analyte molecules. This process, MALDI, is typically used in conjunction with time-of-flight mass spectrometry (TOFMS) and can be used to measure the molecular weights of proteins in excess of 100,000 daltons.
Atmospheric pressure ionization (API) includes a number of methods. Typically, analyte ions are produced from liquid solution at atmospheric pressure. One of the more widely used methods, known as electrospray ionization (ESI), was first suggested by Dole et al. (M. Dole, L. L. Mack, R. L. Hines, R. C. Mobley, L. D. Ferguson, M. B. Alice, J. Chem. Phys. 49, 2240, 1968). In the electrospray technique, analyte is dissolved in a liquid solution and sprayed from a needle. The spray is induced by the application of a potential difference between the needle and a counter electrode. The spray results in the formation of fine, charged droplets of solution containing analyte molecules. In the gas phase, the solvent evaporates leaving behind charged, gas phase, analyte ions. Very large ions can be formed in this way. Ions as large as 1 MDa have been detected by ESI in conjunction with mass spectrometry (ESMS).
ESMS was introduced by Yamashita and Fenn (M. Yamashita and J. B. Fenn, J. Phys. Chem. 88, 4671, 1984). To establish this combination of ESI and MS, ions had to be formed at atmospheric pressure, and then introduced into the vacuum system of a mass analyzer via a differentially pumped interface. The combination of ESI and MS afforded scientists the opportunity to mass analyze a wide range of samples. ESMS is now widely used primarily in the analysis of biomolecules (e.g. proteins) and complex organic molecules.
In the intervening years a number of means and methods useful to ESMS and API-MS have been developed. Specifically, much work has focused on sprayers and ionization chambers. In addition to the original electrospray technique, pneumatic assisted electrospray, dual electrospray, and nano electrospray are now also widely available. Pneumatic assisted electrospray (A. P. Bruins, T. R. Covey, and J. D. Henion, Anal. Chem. 59, 2642, 1987) uses nebulizing gas flowing past the tip of the spray needle to assist in the formation of droplets. The nebulization gas assists in the formation of the spray and thereby makes the operation of the ESI easier. Nano electrospray (M. S. Wilm, M. Mann, Int. J. Mass Spectrom. Ion Processes 136, 167, 1994) employs a much smaller diameter needle than the original electrospray. As a result the flow rate of sample to the tip is lower and the droplets in the spray are finer. However, the ion signal provided by nano electrospray in conjunction with MS is essentially the same as with the original electrospray. Nano electrospray is therefore much more sensitive with respect to the amount of material necessary to perform a given analysis.
In the field of Fourier Transform ion cyclotron Resonance Mass Spectrometry (xe2x80x9cFTICR-MSxe2x80x9d) a Penning ion Trap is used to trap ions. The conventional Penning trap consists of six metal plates forming a cube in a magnetic field (M. B. Comisarow, Adv. Mass Spectrom. 8, 1698(1980), M. B. Comisarow, Int. J. Mass Spectrom. Ion Phys. 37, 251(1981)). Two of these plates (xe2x80x9ctrapping platesxe2x80x9d) reside in planes perpendicular to the magnetic field whereas the other four plates are in planes parallel to the magnetic field. In conventional FTICR-MS, the trapping plates together with the magnetic field are used to trap ions. This is accomplished by applying a small electrical potential (e.g. 1V) to the trapping plates. The remaining plates are held at ground potential. The magnetic field confines ions in the plane perpendicular to the magnetic field line, and the electric field produced by the potential difference between the trap electrodes confines the ions along the magnetic field lines.
Ions in a uniform magnetic field, barring other influences, move in circular orbits (cyclotron motion) with a frequency proportional to ion mass-to-charge ratio (A. G. Marshall, L. H. Christopher, G. S. Jackson, Mass Spectrom. Rev., in press, 1998). However, the presence of an electrostatic field, such as that produced by the trapping plates, produces new modes of motion (magnetron, and trapping) and alters the frequency of the cyclotron motion of the ions. This reduces the resolution of the spectrometer and causes a distortion in the relationship between ion m/z and cyclotron frequency.
The magnitude of the potentials placed on the trapping electrodes is significant both to the degree to which the cyclotron motion is distorted and to the range of the kinetic energy that an ion can have along the magnetic field lines and still be trapped. The kinetic energy of the ions which can be trapped is directly related to the potential on the trapping electrodes and so is the distortion on the cyclotron motion. Thus, in a prior art FTICR cell, the potential on the trapping electrodes would be set as a compromise between trapable ion kinetic energy and distortion in cyclotron motion. The trapping potential must be kept low (e.g. 1V) to avoid excessive cyclotron motion, and as a result, the range of trapable ion kinetic energies is also low (e.g. 1 eV). This limits the FTMS method in its application to external ion sources because such sources often produce ion beams which have a broad range of kinetic energies (R. C. Beavis, B. T. Chait, Chem. Phy. Lett. 181, 479(1991), T.-W. D. Chan et al., Chem. Phy. Lett. 222, 579(1994), J. A. Castoro, C. Koester, C. L. Wilkins, Rapid Commun. Mass Spectrom. 6, 239(1992), C. Koester, J. A. Castoro, C. L. Wilkins, J. Am. Chem. Soc. 114, 7572(1992), J. Yao, M. Dey, S. J. Pastor, C. L. Wilkins, Anal. Chem. 67, 3638(1995), T. Solouki, D. H. Russel, Proc. Natl. Acad. Sci. USA 89, 5701(1992), T. Solouki, K. J. Gilling, D. H. Russel, Anal. Chem. 66, 1583(1994)).
In Laude et al. (xe2x80x9cLaudexe2x80x9d), cylindrical compensation electrodes were inserted between the trap electrodes and the excite/detect electrodes (V. H. Vartanian, F. Hadjarab, D. A. laude, Int. J. Mass Spectrom. Ion Proc. 151, 157(1995)). A reduction in the cyclotron frequency shift of more than 99% was observed.
In the related field of quadrupole mass spectrometry, ions are analyzed via an oscillating electric field (xe2x80x9cQuadrupole Mass Spectrometry and its Applicationsxe2x80x9d, Peter Dawson, ed., copyright 1976, Elsevier Publishing Company, Amsterdam). Typically a quadrupolar electric field is established between four electrodes in the case of a linear quadrupole. Ions are injected into one end of the linear quadrupole and under the influence of the electric field, either pass through to the exit end of the quadrupole or are caused to collide with the electrodes of the quadrupole. By applying the appropriate static and oscillating potentials between the electrodes of the quadrupole, one can select ions of a prespecified mass-to-charge ratio (m/z) to pass from the entrance of the quadrupole to its exit while largely excluding all other m/z ions. Thus, the device acts as a quadrupole mass filter.
The electrodes of a quadrupole mass filter might be designed in many ways. Ideally, four electrodes each having a hyperbolic surface can be used. In theory, electrodes of this form could be used to produce a perfectly quadrupolar electric field. In practice, electrodes of cylindrical geometry are typically used. That is, four cylindrical, rod shaped electrodes are placed symmetrically about the axis of the quadrupole mass filter. This arrangement of electrodes is easier to produce than the hyperbolic electrodes and can be used to produce a close approximation of a quadrupolar electric field.
Alternatively, researchers such as T. Hayashi and N. Sakudo (T. Hayashi and N. Sakudo, Proc. Int. Conf. Mass Spectrom., Hyoto, Japan, 1969 (xe2x80x9cHayashi and Sakudoxe2x80x9d)), and more recently J. Prestage (John D. Prestage, NASA Technical Brief 23(5), pp. 168(May 1999) (xe2x80x9cPrestagexe2x80x9d)) have employed arch shaped electrodes to produce quadrupole mass filters. Such electrode arrangements might also be used to produce close approximations of quadrupolar electric fields. More than four electrodes may be used in such designs. A larger number of electrodes allows for a closer approximation of a quadrupolar field. For example, by employing eight electrodes, Prestage can approximate the quadrupolar field to sixth order. Advantages of this method of producing a quadrupole mass filter include relatively easy production, light weight construction, and less power consumed during operation.
For Example, FIG. 1 depicts the multipole ion guide of Prestage. Shown is multipole ion guide 6 comprising eight electrodes 2 and 4 are arranged symmetrically around a central axis. Four electrodes 2 are grounded, while an oscillating potential of +/xe2x88x92 V is applied between the remaining four electrodes 4. The multipole is extended along its axis and cylindrical in shape with two openings for ions to enter 8 and exit 10 the guide 6. When used as a quadrupole mass filter, the ions enter the guide, and selected ions pass through the device to the exit end 10. To be mass selective, a DC potential is applied between the V+ electrodes and the Vxe2x88x92 electrodes. If no DC potential is applied, the device will simply transfer ions from the entrance 8 to the exit end 10 of the device.
An alternate embodiment of the multipole 6 of Prestage is depicted in FIG. 2. In addition to the multipole of FIG. 1, there are two cylindrically symmetric apertured plates 12 and 14. The apertured plates 12 and 14 are disposed on opposite ends of the multipole. The first apertured plate 14, labeled the xe2x80x9cEntrance Electrodexe2x80x9d is between the ion source(not shown) the entrance 8. The second apertured plate 12 is disposed downstream the first plate 14 and it is labeled the xe2x80x9cExit Electrodexe2x80x9d. By applying a DC offset between these electrodes and the multipole, ions can be trapped in the multipole. Ions would be contained radially by the RF potential applied between the +/xe2x88x92 V electrodes and axially by the potential applied to the entrance and exit electrodes.
A second form of quadrupole mass analyzer is referred to as a quadrupole ion trap (or Paul trap). In contrast to the Penning trap of FTICR MS, the Paul trap does not require and does not use a magnetic field to trap ions. Rather, only an oscillating electric field is used to trap the ions. The Paul trap is a cylindrically symmetric trap composed of three electrodesxe2x80x94a central xe2x80x9cringxe2x80x9d electrode and two xe2x80x9ccapxe2x80x9d electrodes. The two cap electrodes are typically held at the same electrical potential. An oscillating electric field is applied between the cap electrodes and the ring electrode to form a three dimensional quadrupolar field in the interior of the device. Ions can be trapped and manipulated in a variety of ways in this electric field.
Within a quadrupolar electric field, either in a linear device or a three dimensional trap, ions will oscillate with a frequency of motion dependent only on the m/z of the ion. In prior art quadrupole mass analyzers, this characteristic frequency has been used to select, excite, and eject ions from the quadrupole device. In contrast to FTICR MS, ions are detected via a xe2x80x9cchanneltronxe2x80x9dxe2x80x94or other similarxe2x80x94detector rather than by inductive detection. The ions collide with the detector, and are destroyed in the detection process. The inductive detection of FTICR MS preserves the ions because the ions do not collide with the detection device during the detection process.
A third type of related mass analyzer utilizes the Kingdon trap (R. D. Knight, Appl. Phys. Lett. 38(4), 221 (1981)). As suggested by R. D. Knight and later by A. Makarov, the Kingdon trap can be used to trap ions and analyze ions in a one dimensional quadratic electrostatic field. In this case, a central electrode and two xe2x80x9couterxe2x80x9d electrodes are used to generate a cylindrically symmetric electrostatic field of the form:
F=A(Z2xe2x88x92r2/2+Blnr)
Where F is the electric potential, r is the distance from the axis of the trap, z is the position along the axis of the device, and A and B are constants. Clearly from this equation, the field along the axis of the trap is quadratic. Thus, ions will oscillate along this axis with a periodic frequency directly related to the mass-to-charge ratio of the ion. The two outer electrodes are placed opposite one another along the axis of the trap such that the ions oscillate between them with the above mentioned periodic motion. As in the FTICR, ions can be detected via their induced charge on the outer electrodes (A. Makarov, Proceedings of the 47th ASMS Conference on Mass Spectrometry and Allied Topics, 2828(1999)).
Yet another quadrupole ion trap has been disclosed by Micheal W. Senko, Jae C. Schwartz, Alan E. Schoen and John E. P. Syka, Proceedings of the 48th ASMS Conference on Mass Spectrometry and Allied Topics, Jun. 11-15, 2000. Senko et al. disclose a linear quadrupole ion trap comprising a symmetrical arrangement of four detection electrodes and four RF trapping electrodes equally spaced apart around a central longitudinal axis. In the design of Senko et al., each detection electrode is positioned between two RF trapping electrodes, and each RF trapping electrode is positioned between two detection electrodes. Importantly, the electrodes (both detection and trapping) in the Senko et al. design are spaced apart from each other. Such design results in undesirable feedback due to capacitive mismatches as well as RF imbalances.
According to Senko et al., having a truly symmetrically designed quadrupole ion trap will eliminate all feedback detected by the detector from the RF trapping field. This, of course, would require the system be constructed such that it is capacitively matched and that the system be perfectly RF balanced. However, the Senko et al. design is not perfectly RF balanced nor is it capacitively matched. One way Senko et al. attempt to overcome this is by employing high voltage capacitors between each detection and trapping electrode of the system. This too fails to eliminate all of the feedback.
Each of the prior art trapping mass analyzers described above has certain advantages and disadvantages. First, for example, advantages of the FTICR mass spectrometer include high resolution, and mass accuracy, the ability to select ions and perform tandem mass spectrometry (i.e. the selection of ions based on m/z, the fragmentation of the selected ions, and the mass analysis of the fragment ions) on those ions, to detect ions non-destructively, and to detect ions simultaneously across a wide m/z range. Conversely, disadvantages of FTICR include the required use of a strong, highly homogeneous magnetic field, a limited mass range, and limited speed of mass analysis. Second, advantages of the quadrupole mass filter include relative ease of production and use, sensitivity, and quantitation while, disadvantages of the quadrupole mass filter include limited mass range, speed of mass analysis, mass accuracy, and mass resolution. Third, advantages of the alternate design quadrupole mass filters (e.g. as given by Prestage) are potentially further simplified production, lighter weight, and lower power consumed. While disadvantages are lower resolution, mass accuracy, general performance (i.e. the field produced in such a device is not truly quadratic). Fourth advantages of the quadrupole ion trap are the ability to trap and select ions to perform tandem mass spectrometry experiments on the trapped ions, moderate resolution, and moderate mass accuracy. Disadvantages of the quadrupole ion trap are the dependence of mass resolution on scan speed, poor duty cycle (i.e. most ions are lost rather than analyzed, poor trapping capacity) only a small number of ions can be trapped without perturbing the mass analysis. Fifth, advantages of the Kingdon trap are the ability to trap and analyze ions without the need for a magnetic field (as in FTICR) and without the need for an oscillating electrical potential (as used with quadrupole mass filters and quadrupole traps), the ability to detect ions non-destructively, moderate mass resolving power, and potentially the ability to perform tandem mass spectrometry experiments. On the other hand, disadvantages of the Kingdon trap are difficulty of forming and aligning the trap electrodes, complexity of ion introduction into the trap (i.e. ions are trapped only so long as they have a stable orbit about the central electrode) difficulty to excite ions into a coherent axial motion. Yet another disadvantage of the prior art designs includes the existence of undesirable feedback.
The present invention distinguishes itself from prior art by providing a means and method for a novel type of mass analyzer having a unique set of advantages over the above mentioned mass analyzers.
The present invention provides a means and method for a new type of mass analyzer capable of filtering and trapping ions with specific advantages over prior art mass analyzers.
In the prior art multipole design according to Prestage (FIGS. 1 and 2), eight electrodes are arranged symmetrically around a central axis. Four of these are grounded and an oscillating potential of +/xe2x88x92 V is applied between the other four electrodes. According to Prestage, the multipole is extended along its axis such that it is substantially cylindrical in shape. Being cylindrical, the device has two openings, xe2x80x94one at each endxe2x80x94and the device can be used as a quadrupole mass filter. In this case, ions enter one end of the multipole and only selected ions pass completely through the multipole to its exit end. If the multipole is to be mass selective, then a DC potential must be applied between the V+ electrodes and the Vxe2x88x92 electrodes. Alternatively, if no DC potential is applied, the multipole may be used as an ion guide (i.e., simply to transfer ions from the entrance end to the exit end of the device).
According to the present invention, the electrodes which were grounded according to Prestage are held only nominally at 0 volts. For example, these electrodes are each independently connected to ground through a 1 megaohm (Mohm) resistor. These nominally grounded electrodes are then used to detect trapped ions via charge induction in the manner of Fourier Transform Ion Cyclotron Resonance (FTICR) Mass Spectrometer. More particularly, ions are first cooled to the center of the trap via collisions with the rest gas. During subsequent ion excitation and detection, the trap is substantially free of gas. The ions are then excited by applying a broadband excitation pulse between the V+ and Vxe2x88x92 electrodes. This broadband excitation pulse is applied so as to induce the ions to orbit about the axis of the ion trap in coherent ion packets. While ions might be distributed along the length of the trap, substantially all ions of a given m/z should be at about the same angular position in their orbits at the same time. Further, ions of a given m/z will have a given frequency of motion about the central axis of the trap. As in conventional FTICR, by measuring the frequency of the signal induced on the detection electrodes, the m/z of the ions can be determined, and by measuring the amplitude of the induced signal, the relative number of ions of that given m/z can be determined.
In an alternate embodiment of the linear multipole trap according to the present invention, a central set of electrodes and two trapping electrodes (instead of the DC trap electrodes) may be used. The trapping multipoles are held at a slightly higher DC potential than the central analysis multipole (e.g., 2V). This DC offset between the multipoles serves to trap ions in the analysis multipole (i.e., the central electrodes). At the ends of the analysis multipole, the oscillating quadrupolar field is not greatly perturbed and therefore the motion of the ions at the center of the multipole is therefore substantially the same as the motion of ions near the ends of the analysis multipole. The RF electrodes of the trapping multipoles and analysis multipoles are all driven by the same RF driver. Therefore, the RF electrodes will all have the same potentials and frequencies applied to them, and the RF electrodes of the analyzing multipole are capacitively coupled to their counterparts in the trapping multipoles.
Yet another embodiment of the linear multipole trap according to the invention may comprise only a single multipole with the detection electrodes divided into three sections to achieve the same effect. That is, the central section is the xe2x80x9canalyzingxe2x80x9d section, whereas the two outer sections are the xe2x80x9ctrappingxe2x80x9d sections. The regions of the detection electrodes defining the trapping section of the multipole are not used to detect ions xe2x80x94rather, these electrodes are held at a high DC potential with respect to the central detection electrodes, which tends to repel the ions back into the analyzing section. The combination of this DC field and the RF field generated by the potential applied between the RF electrodes, traps ions within the analyzing section of the multipole. The advantage of this embodiment is that, without regard to mechanical tolerances, the RF field is guaranteed to be homogeneous throughout the multipole (i.e., there is no RF electric field component along the axis of multipole and the RF field experienced by an ion is not dependent on its position along the axis of the multipole).
In a mass spectrometer employing the preferred embodiment of the linear multipole trap according to the present invention, ions may be generated at an elevated pressure (e.g., atmospheric pressure) via, for example, electrospray ionization. Ions are transferred, by entrainment in a gas flow, through a capillary from the atmospheric pressure region into a first pumping region. Some of these ions pass through the first pumping region and into a second pumping region through a skimmer. In the second pumping region, ions enter a first trapping section of the multipole. The pressure in the second pumping region is such that ions undergo sufficient collisions with the gas in the first trapping section of the linear multipole trap to be cooled to near room temperature (e.g., 10xe2x88x922 mbar). Having been cooled to near room temperature, the ions are allowed to pass into the analysis section. This third pumping region is pumped to a lower pressure than the second pumping region, such that the ions have a large mean free path.
Other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of the structure, and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following detailed description with reference to the accompanying drawings, all of which form a part of this specification.